This invention relates to the processing of extrudable materials and in particular provides a screw processor having means for controlling the generation of shear within the apparatus.
Screw processors typically comprise one or more screw threads mounted for rotation within a housing. Material to be processed is fed into the housing and is conveyed to an outlet by rotation of the screw or screws. At the outlet, the processed material is normally discharged through a die. Material contained in the processor is subjected to shear forces generated by reaction between the screw thread and the housing and also between interacting screw thread portions, where twin screws are employed. The action of the screw thread on the material is also to increase its pressure so that at the outlet, pressures of the order of 7 to 360 bar are not exceptional. The material may also be heated during passage through the screw processor and this may be due to the use of external heaters, arranged to heat the housing as the material passes from the inlet to the outlet, but may also be heated by frictionally-generated heat.
Conventional screw processors are driven by a gear box at one end, arranged to drive a shaft which is supported in a bearing, located at the inlet end of the housing. Because of the difficulties of providing passage for the extruded material to pass around a bearing at the outlet end, the shaft or shafts bearing the screws are normally supported in cantilever fashion within the housing. As a result, the shaft is subjected to radial forces which cause the shaft to bend so that the shaft engages the housing or barrel during the mixing of extrusion process. This causes abrasive metal-to-metal contact and also generates high rates of wear between the screw flights and the inner surfaces of the barrel. As a result of the contact between the shaft and the barrel, shear forces are generated in an unpredictable and variable manner within the housing. When processing certain materials, particularly starchy products, undesirable changes or damage can occur under conditions of uncontrolled shear forces. For example, in the processing of cereal starches into, for example, breakfast cereals, snacks and crispbread, or the conversion of vegetable protein into meat-like products, undesirable changes and degradations of the raw materials can be caused by undesirably high and localized shear forces.
Typically, starch cells may be at least partially ruptured and the polysaccharides in the starches may be degraded into shorter oligosaccharides such as maltose. As a consequence, color and flavor can be adversely affected. If the degree of shear applied to the materials as they are processed could be controlled, undesired degradation could be avoided or a desired degree of transformation in the product could be achieved. Whereas it is theoretically possible to control the amount of shear by changing the rotational speed or geometry of the screws, in practice, this is very difficult to achieve because of the distortion of the screws which occurs in a conventional screw processor. The present invention seeks to control the amount of shear applied to a material to be processed, particularly a food material, in order that the desired degree of transformation of the product can be secured during the extrusion process. In the case of, for example, starchy materials, the amount of starch cell rupture can be varied as desired from essentially nil to treatments where starch cells are swollen but not ruptured, through to treatments where at least some cells are ruptured and ultimately to the complete destruction of the starch structure.